The present invention relates to air-to-air heat pumps, and more specifically to improved refrigeration circuits for controlling operation of such heat pumps.
Conventional heat pumps include a refrigeration circuit with a compressor and indoor and outdoor heat exchanger coils which function alternately as a condenser and an evaporator in response to a thermostat controlled valve reversing the direction of refrigerant flow between heating a cooling cycles. During cooling cycles the indoor coil functions as an evaporator, absorbing heat from indoor air, and the outdoor coil functions as a condenser, rejecting heat into the outdoor air. Conversely, during heating cycles the outdoor and indoor coils function as evaporator and condenser, respectively, absorbing heat from outdoor air and rejecting heat to indoor air for comfort heating.
The outdoor coil and associated heat exchange fins are subject to collection of frost and/or snow when weather conditions so dictate. This results, of course, in reducing heat transfer by blocking air flow through the fins, and by its insulating effect on the fin and coil surfaces. Frost and/or snow is periodically removed by switching the system to operate temporarily in the cooling mode, causing the higher temperature refrigerant to flow to the outdoor coil, raising the temperature thereof sufficiently to melt the frozen or crystallized moisture. Such temporary cooling or defrost cycles are commonly initiated by means responsive to the thickness of frost, snow or ice buildup, in cooperation with timer means.
During defrosting cycles condensed refrigerant is delivered from the outdoor coil to the indoor coil through the cooling capillary tube sized for optimum refrigerant flow under the high condensing pressure of cooling cycles. Under the reduced condensing pressure of defrost cycles the refrigerant flow is restricted by the capillary tube to a flow volume less than the rate of condensation in the outdoor coil. Therefore the system liquid refrigerant is collected in the outdoor coil leaving the indoor coil dry. The compressor continues running while its suction pressure reduces the pressure in the indoor coil to less than zero.
The defrost cycle is terminated, and heating cycle resumed, in conventional systems, in response to the opening of thermostat contacts when the frost, etc. has been removed, or at the end of a predetermined time period. The resulting sudden delivery to the compressor crankcase of liquid refrigerant which has been expanded in the indoor coil, then condensed and retained in the outdoor coil during the defrost cycle, would be damaging to the compressor mechanism if suddenly returned to the indoor coil at the beginning of a heating cycle. The conventional approach to guarding against such potential damage is to install a trap-type accumulator between the reversing valve and the compressor suction port for collecting the surge of liquid refrigerant, and gradually releasing it into the compressor crankcase at a safe, metered rate.
The use of a trap-type accumulator, while considered necessary for protection of the compressor, is generally recognized to have a number of disadvantages. These disadvantages are enumerated in applicant's prior U.S. Pat. No. 4,266,405, which discloses an improved heat pump refrigeration circuit which bypasses the accumulator during cooling and defrost cycles. However, the accumulator is still required, and refrigerant flows therethrough during normal heating cycles. This adds to the cost and detracts from the efficiency and potential capabilities of the heat pump system.
Accordingly, it is a principal object of the present invention to provide an air-to-air heat pump which operates efficiently and without danger of damage to operating components, yet does not include an accumulator.
Another object is to provide a heat pump system wherein defrost cycles are initiated and terminated by means independent of conventional thermostat control.
A further object is to provide means for minimizing the time length and maximizing the dependability of control of defrost cycles in air-to-air heat pump systems.
In a more general sense, the object of the invention is to provide an air-to-air heat pump with improved heat gain during cold weather, to the extent of replacing combustion heating at average temperatures in all climates, and automatically adding supplementary electric heating during periods of abnormally low temperature.
Other objects will in part be obvious and will in part appear hereinafter.